1. Field of the Invention
The present invention relates to a surface acoustic wave device having an electrode structure with high dielectric strength at high frequencies and also relates to a process for manufacturing such a surface acoustic wave device.
2. Description of the Related Art
Surface acoustic wave devices are electronic components using surface acoustic waves, which are mechanical vibrations propagating along surfaces of solid materials, and are used for filters, resonators, and duplexers.
Recently, the miniaturization of mobile communication terminals such as mobile phones has been advancing. Therefore, electronic components of such terminals must be reduced in size.
A surface acoustic wave device includes a piezoelectric substrate and a pair of interdigital electrodes (interdigital transducer (IDT) electrodes), disposed thereon, containing a conductive material having low density. The interdigital electrodes each have electrode fingers that are alternately arranged. Since the surface acoustic wave device has such a simple configuration, the surface acoustic wave device is fit for filters, resonators, and duplexers that must be reduced in size when used for such mobile communication terminals.
A known surface acoustic wave device usually includes interdigital electrodes principally containing Al or an Al alloy having high conductivity and low density, as disclosed in Japanese Unexamined Patent Application Publication No. 2001-94382 (hereinafter referred to as Patent Document 1).
When such a surface acoustic wave device is used as, for example, an antenna duplexer placed in a radio-frequency (RF) section to which high voltage is applied and which is placed downstream a transmitting amplifier, the surface acoustic wave device must have high dielectric strength. Furthermore, since the mobile communication terminals have recently used higher frequencies, the operating frequency of the surface acoustic wave device must be increased from several hundred MHz to several GHz.
In order to increase the frequency, the electrode fingers of the interdigital electrodes of the surface acoustic wave device must be reduced in size and distance between the electrode fingers. For example, bandpass filters with a center frequency of 2 GHz must have a width of about 0.5 μm and bandpass filters with a center frequency of 10 GHz must have a width of about 0.1 μm.
When high-voltage signals are applied to the interdigital electrodes having such fine electrode fingers, the interdigital electrodes suffer from strong stress due to surface acoustic waves. The stress exceeding the critical stress of the interdigital electrodes causes stress migration. The stress migration is such a phenomenon that metal atoms contained in the interdigital electrodes migrate through grain boundaries and/or stable faces of crystals and thereby voids are formed in the interdigital electrodes and/or hillocks are formed thereon. The stress migration causes a breakage of the electrodes, electrical breaks, an increase in insertion loss of elements, a decrease in Q-factor of resonators, and the like, and finally causes a deterioration in property of the surface acoustic wave device.
In particular, the known surface acoustic wave device, disclosed in Patent Document 1, including the interdigital electrodes principally containing Al or an Al alloy has high resistivity and a low melting point. Therefore, when the interdigital electrodes are reduced in size, there is a problem in that the electrical resistance is greatly increased and the resistance to stress increased due to high frequencies is lowered, thereby causing the stress migration to occur.
Another type of interdigital electrodes have been disclosed and such interdigital electrodes principally contain Cu or a Cu alloy instead of the Al or Al alloy.
For example, Japanese Unexamined Patent Application Publication No. 2002-26685 (hereinafter referred to as Patent Document 2) discloses an interdigital electrode for surface acoustic wave devices. This interdigital electrode principally contains Cu or a Cu alloy having low electrical resistance and high resistance to stress migration. Patent Document 2 also discloses a configuration in which a first electrode layer containing Ti or a Ti alloy is placed between a piezoelectric substrate and a second electrode layer containing Cu or a Cu alloy such that the orientation of crystals in the second electrode layer is improved and the second electrode layer is securely joined to the piezoelectric substrate.
In the configuration disclosed in Patent Document 2, since the second electrode layer is directly disposed on the first electrode layer, Ti diffuses into the second electrode layer and Cu diffuses into the first electrode layer at the interface of the first and second electrode layers. Therefore, there is a problem in that such diffusion causes an increase in electrical resistance of the interdigital electrode. When surface acoustic wave devices include interdigital electrodes having high electrical resistance, the surface acoustic wave devices have a large internal loss and the stress migration is apt occur in the surface acoustic wave device.